Homogenizing out-of-plane cation composition in perovskite solar cells

Perovskite solar cells with the formula FA1−xCsxPbI3, where FA is formamidinium, provide an attractive option for integrating high efficiency, durable stability and compatibility with scaled-up fabrication. Despite the incorporation of Cs cations, which could potentially enable a perfect perovskite lattice1,2, the compositional inhomogeneity caused by A-site cation segregation is likely to be detrimental to the photovoltaic performance of the solar cells3,4. Here we visualized the out-of-plane compositional inhomogeneity along the vertical direction across perovskite films and identified the underlying reasons for the inhomogeneity and its potential impact for devices. We devised a strategy using 1-(phenylsulfonyl)pyrrole to homogenize the distribution of cation composition in perovskite films. The resultant p–i–n devices yielded a certified steady-state photon-to-electron conversion efficiency of 25.2% and durable stability.


Synthesis of 1-(phenylsulfonyl)pyrrole (PSP)
PSP was synthesized according one step reaction of benzenesulfonyl chloride and pyrrole.For example, 2.2 mL pyrrole, 1.0 mg tetrabutylammonium bisulfate and 5.6 mg sodium hydroxide were dissolved in toluene (100 mL) in an ice bath.A mixture comprising 6.2 mL benzenesulfonyl chloride and 50 mL toluene was slowly dropwise added in pyrrole solution.
We expected an appropriate geometrical size to ensure molecules to stay in the films rather than escape because of there would be residuals in the films to maintain inhibiting cations inhomogeneity, and compensate discrepancy of cations size in perovskite.Considering sulfoxide group is recognized to serve as electron doner for PbI2, according to PbI2 lattice space of 6.32 Å (Calculated using Bragg's Law  =  2sin() = 1.54 2×sin(( 13.98°2 )) = 6.32Å ), inserting two single five-or six-membered rings is most suitable to creat a stable structure.
Furthermore, the directional electrostatic potential (ESP) distribution and moderate negative potential center enable additive to precisely regulate the target site during crystallizations and stable adsorption to perovskite (Supplementary Fig. 2).Therein, it is preferrable for two asymmetry single rings to be arranged on either side of the sulfoxide group.We calculated local potential of perovskite lattice adsorbed with PSP in order to assess additive properties through potential variation at specific region.The results demonstrated a significant potential variation at Cs region, suggesting PSP prefer to adsorb at Cs region through charge interaction (Supplementary Fig. 3).Supplementary Fig.   that serious behavior of cations accumulation, leading to Cs-rich phase.The conventional XRD measurements results demonstrated that, with the increasing x value, the right-side shoulder peak was growing stronger and shifting towards higher 2-theta degrees.A linear relationship between Cs/(Cs+FA)-ratio and the location of shoulder peaks was observed, which is consistent with the assumption that the Cs-rich phase preferred to accumulate at the bottom region within perovskite films, thus leading to a gradient phase distribution of Cs-poor to Cs-rich evolution from the surface to bottom.During the annealing process, PSP residuals should be a high-temperature crystal structure remain in the perovskite films, which possibly accounts for the passivation effect.However, we cannot detect PSP related characteristic XRD signals in perovskite films, it was possibly attributed to tiny amount of PSP introduced.We presume that PSP have different function interact with perovskites through different structures.At the stage of crystallization and perovskite phase transition, PSP worked as homogeneous phase regulator through RT structure.
Regarding with the as-fabricated films, PSP residues with high-temperature structure remains in perovskite films possibly worked as passivator and further stabilize the device through strengthen the bottom region.

. 7 .
(a) Conventional XRD results of a series of FA1-xCsxPbI3 perovskite films with varied x value.The enlarged plots focusing on (b) the (100) and (c) the (200) plane.

. 8 .
Plot concluded fron the conventional XRD results, depicting relationship between Cs ratio in perovskite and the corresponding peaks position.Two perovskite dominant peaks of were counted in the graph.Peaks position of perovskite (100) plane (black line in the plot) demonstrated an exponential linear relationship, which is associated with undersized Cs atoms would contract perovskite lattice.And peaks position of (200) plane (orange line in the plot) leaps up between x=0.05 and 0.1, together with significant shoulder peaks can be observed after x=0.1.There might be a threshold value around x=0.5

a
Supplementary Fig. 9. (a) Conventional XRD patterns detected from the top surface of perovskite films with different amount of PSP introduced.The enlarged plots focusing on the (b) (100) plane and (c) the (200) plane.

10 . 11 . 12 .. 14 .
Results of in-situ grazing synchrotron radiation grazing incidence wide angle X-ray scattering (in-situ GIWAXS) mesurements for the perovskite films of (a) the reference, (b) PSP-1.2,(c) PSP-2.4 and (d) PSP-4.8.The in-situ GIWAXS results could clearly reveal the two critical process during perovskite formation, which have been proposed by numerous studies: (I) ions mixture in precursor interact to form non-photoactive δ-phase; (II) phase transition from intermediate δ-phase to objective α-phase perovskite.It was an essential approach to identify the complicated crystalize progress of perovskite films, then reveal how vertical FA-Cs phase segregation arises.SEM images of the surface morphonology of perovskite films treated with varied amount PSP: (a) the reference, (b) PSP-1.2,(c) PSP-2.4 and (d) PSP-4.8.The scale bar was 500 nm.Simulated curve of penetration depth variation on the FAPbI3 perovskite films regarding the X-ray beam of 13.035 keV.X-ray photoelectron spectroscopy (XPS) depth profiles of (a) the reference sample and (b) the PSP sample.

Supplementary Fig. 15 . 17 .
Light-intensity (P) dependent (a) VOC and (b) JSC variation plots of the reference and the PSP devices.The slope was fitted using equation of   =  () (K is Boltzmann constant, q is elementary charge and T is temperature) and   ∝   , respectively.The fitted slope values were noted inside the figure.Supplementary Fig. 16.I-V curves measured of solar cells in dark condition.The results of light-intensity dependent VOC variation and leakage current measured from solar cell configuration demonstrated inhabitation of non-radiative recombination.Trap density of states (tDOS) plots extracted from thermal admittance spectroscopy (TAS) of the PSCs with and without PSP.

18 .
(a) XRD patterns of PSP powder at room temperature (denote as RT), and powder after heated at 100 ℃ for 30 minutes (denote as 100 ℃).(b) Thermogravimetrydifferential scanning calorimetry (TG-DSC) curves of PSP powder, the temperature ranges from room temperature to 100 ℃ and further cooled down to room temperature.The black arrow points the peak indicating structural transition of PSP.We found that PSP undergoes a spontaneous self-structure transform under thermal stress, and has two typical temperaturedependent crystal structures, as shown in XRD results.The precise temperature of PSP structure transition was determined as around 90 ℃ concluded from the TG-DSC results.

Table 2 .
Summary plot of the reported the state-of-the-art efficiencies of PSCs at that time, showcasing both p-i-n and n-i-p configuration.Solid circles represent efficiencies that have been certified, while hollow circle denotes uncertified efficiency.angles,which was calculated using equation τ1/e=sin(θ/μ) (τ1/e is the depth at which the intensity of radiation on a material is attenuated to 1/e (37%) and μ is the linear absorption coefficient).Summary of computation energy evolution details for the processes of different perovskite crystallization and phase transition.The reaction coordinate of 0, 6 and 12 were indictive for the AX-PbI2 (w.PSP), δ-APbI3 (w.PSP) and α-APbI3 (w.PSP), Supplementary Fig. 25.Statistical chart presenting detailed PV parameters, which include (a) VOC, (b) JSC, (c) FF and (d) PCE.The chart illustrates the variation in these parameters for PSCs treated with different amounts of PSP.

Table 6 .
Summary table of the reported state-of-the-art efficiencies of perovskite solar cells.